Requirements exist for a retaining clip or fastener of small size which is capable of reliably and repeatedly exerting a high, holding force. A particular embodiment of these requirements is the arming wire retaining clips used in fuzes of aerial-delivered ordnances wherein a wire is passed through the fuze to maintain a safe, unarmed condition. To prevent the wire from being inadvertently removed, retaining clips are placed upon the wire on both sides of the fuze. These clips must exert a high, gripping force to preclude removal during normal ordnance handling and aircraft maneuvers. Furthermore, in the event that the clips are removed and reused, for example, when ordnance items are loaded but not deployed and are subsequently reloaded, the clips must exert the same, consistent high retaining force without structural failure. The requirements of small clip size plus a consistent, repeatable and reliable applied load are necessary to assure a fail-safe and operable fuze. The clip must be able to apply a load of 75 to 100 pounds on the arming wire, which is equivalent to 25 to 50 pounds of pull on the wire to remove it from the clip. Additionally, the clip must be small to minimize wind resistance. Furthermore, from reliability and safety considerations, the load applied by the clip must be repeatable with a high level of assurance.
Some currently-used clips are a variation of the brass Fahnestock clips. This clip was designed so that a pull of 20 to 30 lbs. would remove the arming wire. However, with this design, compression of the clip to its mechanical stop results in stresses well above the elastic limit of the material. Therefore, if the clip were compressed and the released, the original and final, no-load position would not be the same. The resulting permanent set causes a reduction in the force applied to the arming wire, resulting in a reduction of the pull force required to remove the arming wire from the retaining clip. This reduction is not constant but dependent upon the amount of permanent set, the actual stresses and the material yield strength resulting from heat treatment. It is anticipated that the reduction of wire pull load could be as much as one-third of the original load, raising the question of the reliability and repeatability of this clip.
Due to the severity of the bends, the inability to completely deburr the Fahnestock clip and the fact that the clip is worked far beyond the material's elastic limit inherently makes this design susceptible to stress corrosion and subsequent failures. In addition, at the root of the hook which serves as a catch to secure the arming wire, a critical condition of stress concentration will occur, invariably shortening the life of the clip by stress corrosion and ultimate failure.
An alternate design of the above-described Fahnestock clip is the beryllium-copper scissor clip from which wire pull loads of 20 to 25 lbs. can be obtained. As with the above clip, the maximum, sustained stresses are beyond the elastic limit, resulting in the clip being permanently deformed. Again, this deformation, or set, alters the loading applied to the wire by the clip which in turn affects the reliability and repeatability of the clip in use.